Two Australian scientists believe they have found evidence of a parallel universe of strange matter within our own Solar System.

Dr Robert Foot and Dr Saibal Mitra report that close-up observations of the asteroid Eros by the Near-Shoemaker probe indicate it has been splattered by so-called "mirror matter".

Mirror matter is not anti-matter, it is altogether weirder. It is somehow a "reflection" of normal matter, a sort of parallel series of particles required to restore the balance of the Universe.

Sounds far-fetched - some believe so. However, experiments are underway to confirm or deny the existence of this strange, potentially significant but as yet undetected component of the cosmos.

Cosmic balance

Mirror matter is a hypothetical form of matter that restores nature's flawed left-right symmetry.

Laws of nature, such as the rules that govern the interactions of fundamental particles, show a high degree of symmetry except that some laws are not the same when reflected in a hypothetical mirror.

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This means that elementary particles display a preference for left over right. In a way, the Universe is left-handed. Why? Nobody knows.

Many physicists are happy with this idea believing that in the first instants of the Big Bang everything was perfectly symmetrical. Only when the cosmos cooled did it become asymmetric, with a difference emerging between left and right.

But some scientists do not accept this. They maintain that the Universe has a left-right balance because there exists "mirror matter" - for every known particle there is a mirror particle that restores the cosmic balance.

Dark matter

Mirror matter would produce its own light but we would not be able to see it because mirror matter only interacts with our matter via gravity.

Dr Robert Foot believes that mirror matter would have been made in abundance in the Big Bang and that it is all around us but we can't see it.

Mirror man Dr Robert Foot

"There could be mirror matter stars, planets and galaxies out there," he told BBC News Online.

"In fact, some think that the unseen so-called "dark matter" of the Universe could actually be mirror matter," he adds.

"Mirror matter is perfect to explain dark matter. It's dark and can only be detected through its gravity."

Dr Foot believes he has found evidence that it is here, closer than we believed, and that it had had a measurable effect on our spaceprobes.

Mysterious force

In October 2000, the Near-Shoemaker spacecraft lightly touched down on the 13-by-13-by-33-km (8 by 8 by 20 miles) Eros asteroid. It was the first time a probe had landed on an asteroid.

Its close scrutiny of Eros revealed many strange features - such as flat-bottomed craters filled with a peculiar bluish dust, and a puzzling lack of small craters.

He calculates that small objects containing mirror matter could have struck the asteroid and left behind precisely the same scars that are seen. Indeed, he says there is no other credible explanation.

He also calculates that mirror matter may explain the mysterious force that acts on both the Pioneer 10 and 11 deep spaceprobes.

Distant probes

Launched in 1972, the Pioneers are leaving the Solar System in opposite directions. Detailed analysis of their trajectory indicates that they are both subject to a tiny, unexplained force that is slowing them down.

Dr Foot believes that mirror matter exerting a drag on the Pioneers could be to blame.

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"How else can you explain that both Pioneers, on opposite ends of the Solar System, experience the same force pushing in the same direction?" Dr Foot asks.

In a research paper to be published shortly, Drs Foot and Mitra suggest that mirror matter may even have struck the Earth.

He singles out three possible events: the 1908 Tunguska impact in Siberia and low-altitude, low-velocity fireballs seen in Spain in 1994 and in Jordan in 2001.

"Mirror matter could also explain these events," he told BBC News Online.

Future experiments

Many scientists dismiss mirror matter as wild speculation but even the sceptics will have cause for thought if the latest experiments from the European Centre for Nuclear Research (Cern) are to be believed.

Experiments involving so-called ortho-positronium - an arrangement in which an electron orbits a positron (its antimatter equivalent) - show that it decays slightly faster than can be explained.

This could be due, says Dr Foot, to the electrons changing fleetingly into mirror matter and then back again.

Experiments at Cern and in Moscow hope to determine in the next year or so if mirror matter really does exist.

Dr Robert Foot is from the University of Melbourne; Dr Saibal Mitra is from the University of Amsterdam.